Insulating a Ceiling with Rigid Foam

I have an exposed 2x6 tongue-and-groove ceiling with 4x12 rafters. This is a seasonal room that I’m going to heat on different occasions. I’m going to remove the shingles and I thought I would apply 2 inches of rigid foam on the outside with ½-inch OSB sheathing on top, then 30 lb. felt and shingles. Do I need an airspace of ¾ inch between the foam and the OSB? I live in Michigan.

You’re certainly on the right track. Leaving the interior alone and concentrating your efforts on the roof allows you to insulate effectively without wrecking the room’s charm.

Rigid foam insulation also is a smart choice. Settled R-values range up to about 6.5 per inch and when installed properly the foam can be a good air and moisture barrier.

So, to answer the two remaining questions: Is 2 inches of insulation enough? And should you vent the roof?

Let’s start with how much insulation you should install. If you plan on heating the room only for special occasions, like the big holiday bash for your yodeling club, 2 inches of foam would probably be fine. You’d be able to keep the room comfortable as long as you didn’t mind cranking up the heat and burning a little extra fuel.

But if you want to use the room for longer stretches, that’s not enough. In Michigan, the Department of Energy recommends that cathedral ceilings be insulated to R-38, nearly three times the insulating value you get from 2 inches of polyisocyanurate foam.

Unless you heat the room with some form of renewable energy (wood, solar or wind), the cost of keeping it comfortable only increases with time. Anything you invest in insulation now saves money over the long term, and if you decide to use the room more often you won’t have to retrace your steps and add more insulation.

To me, it would make sense to at least meet the DOE recommendations and beat them if possible. You could do that with thicker foam, by hiring someone to spray in closed-cell foam between a new set of rafters before you add sheathing, or by contacting a local company that deals in structural insulated panels (SIPs) and seeing what’s available for roof panels.

A good rule of thumb with insulation: More is better.

Venting the roof is another question. That’s been standard practice for years. An air space beneath the sheathing along with soffit and ridge vents create a “cold roof” that is supposed to wick away moisture and prevent the formation of ice dams along the eaves.

More recently, many builders have successfully skipped the vents. The key is to prevent the movement of warm, moist air into the roof where it could condense and lead to the growth of mold or decay.

With a sufficiently thick layer of insulation and effective air and moisture barriers, ventilation isn’t necessary in most climates. One beauty of closed-cell foam is that it creates its own air and moisture barrier because it fills even very tiny gaps and has a very low “perm rating.”

But if you’re only adding a couple of inches of insulation, ventilation is a good idea even if you place a vapor barrier over the roof before adding the foam. You might look at a product from Atlas Roofing called ACFoam CrossVent, which combines foam insulation, OSB sheathing, and an air space in a single roof panel.

None of this is especially cheap. Then again, neither are energy costs.

About the AuthorAn accomplished woodworker and carpenter, Scott Gibson is the former editor of Fine Woodworking magazine, and a former editor at Today's Homeowner and Fine Homebuilding magazines. He also is former managing editor of the Kennebec Journal, a daily newspaper in Maine.